Process for desulphurizing mineral oil distillates



April 19, 1938b D. s. MoKlTTRlcK 2,114,852

PROCESS FOR DESUIJPHURIZING MINERAL OIL DISTILLATES Filed Aug. 11, 1936 (ana/enfer aienied r. l, litt-938 OIL DIST'ILLATES Donald s. Merrimack, oakland, cam.. mimmto Shell Development Company, San Francisco,

Calif., a corporation of Delaware Application August l1, 1936, Serial No. 95,385

. i 16 Claims.

This invention pertains tothe treatment of hydrocarbon oils for the removal Aof sulphur, and is .particularly concerned with an improved process for desulphurizing cracked distillates, although similar distillates` from other sources, such as straight-run distillates and polymerid oils, may also be desulphurized in accordance with the present invention.

More specifically, the present vprocess is 'concerned with the removal of the more refractory sulphur compounds, such as thiophene and its homologues, particularly alkyl thiophenes, from lower boiling distillates, such as gasoline, kerosene, gas oil, and cracked naphtha, but it may be applied for the removal of other sulphur compounds, and may, moreover, be applied to the treatment of higher-boiling hydrocarbon fractions.

'Ihe low-boiling hydrocarbon fractions produced as a result of theV application of heat to higher boiling hydrocarbon oils or shales 'contain objectionable sulphur compounds, such as hydrogen sulphide, mercaptans, aliphatic and cyclic thioethers land thiophenes. Certain of these sulphur compounds are readily removed or rendered innocuou. by simple methods of treatment, `such as extraction with solvents and conversion into disulphides. Other sulphur compounds, particularly thiophene and the alkyl thiophenes, are diicult tov remove except by intensive methods which concurrently destroy or re'- move unsaturated hydrocarbons. Since cracked distillates contain large amounts of unsaturated hydrocarbons, such as oleflnes and di-oletlnes, these methods have heretofore'been uneconomical, and have not been applied to the treatment of vsuch distillates save when hydrogenation or a similar conversion of the unsaturated hydrocarbons was at the same time an object of the treatment. The sulphur compounds most diilcult to remove from pressure distillate without adversely affecting the yield or knock rating of the gasoline are those occurring in the upper ends of the gasoline oiling above about 140 C., and appear to be polymethyl `and higher derivatives of thiophene.

In the present specificationv and claims the word "thiophenes is used to designate thiophene'and its homologues; and the expressions unsaturated aliphatic hydrocarbons and aliphatic unsaturates are used to designate open or closed chain hydrocarbons capable of reacting with halogens by addition, i. e., oleiines, diolenes, and similar hydrocarbons.

Pressure distillate consists chieiiy of a mixture (Cl. ISG-24) of iive hydrocarbon types, parafdns, naphthenes, aliphatic open chain and cyclic unsaturates, and aromatics. The solubility of these types in polar solvents increases in the order given. Thiophene homologues and other sulphur compounds have properties very similar toaromatics, in that they are preferentially soluble inselective solvents, but they exhibit these propertiesto a more intense degree. It has heretofore been proposed to utilize this property of sulphur compounds in the desulphurization of fractions by distilling the sulphur-'bearing fraction in the` presence of a polar solvent. whereby a wholly or partially desulphurized distillate is obtained and the Sulphur compounds are concentrated in the distillation residue. (See the patent to Roelfsema No. 2,069,329.) 'I'he separation attainable between sulphur and hydrocarbons, is, of course, dependent upon the difference between the deviations of the sulphur compounds and the hydrocarbons fromRaoults law. I` have `found that this difference in deviations 'of aromatic hydrocarbons and sulphur compounds, particularly thiophenes, becomes less as the molecular weights of the aromatics and sulphur compounds increase, and that the emcient desulphurizatlon of fractions boiling above140 C. with a good yield isdiilcult; unless the distillation4 column is operated to distlll over less than 'Z0-85% ofthe initial charge.

In accordance with the present invention it was found that distillates containing such refractory sulphur compounds and aliphatic unsaturates can be eectively desulphurized, or that the objectionable sulphur compoundscan be effectively converted into innocuous sulphur compounds by distilling the initial" fraction in the presence of a polar solvent or mixture of solvents which preferentiaily dissolves sulphur compounds to produce a distillate containing all or the greater portion of the aliphatic unsaturates, and little or no sulphur, and a, residue containing substantially all of the sulphur'and a major portion of the aromatics and then subjecting the residue to a desulphurization treatment to cause the removal or conversion of the sulphur compounds.

The desulphurized residue may then be blended the following detailed description, taken in connection with the accompanying drawing, which is a schematic flow diagram illustrating one speciilc mode of practicing the present invention.

A feed mixture of cracked hydrocarbons and sulphur compounds boiling, for example, within gasoline boiling range which may, if desired, have been given a preliminary treatment with sulphuric acid or with an alkaline solvent to remove the less refractory sulphur compounds, is fed through the valve I, mixed in the mixer 2 with a polar solvent supplied through the valve 3 from a solvent tank 4, and fed into the fractional distilling column 5 through a conduit 6. The column may be provided with the usual contact means, such as bubble trays, and with heating means, and the apparatus may be provided with pumps, gauges, heaters, heat exchangers, and other adjuncts not shown. In the column 5 the feed mixture is distilled to produce a distillate Withdrawn through line 'I containing the greater part or all of the aliphatic unsaturated hydrocarbons, such as olefmes, besides solvent and hydrocarbons which are less soluble in the solvent than aliphatic unsaturated hydrocarbons, but little or no sulphur. The distillate may, for example, contain between 50 and of the original hydrocarbon mixture, and may contain from 0 to about r70% of the aromatics, although even a higher proportion of hydrocarbons and aromaics may at times be removed in the distillate, depending upon the solvent employed and the composition and boiling range of the original oil. As a specific example, when treating a cracked gasoline fraction containing about 13% aromatics, 1% sulphur, and boiling between and 200 C., and employing ethylene diamine as the polar solvent, I may produce a distillate containing between 70 and 80% of the initial hydrocarbons, between about 35 and 50% of the initial aromatics, and about 0.06% sulphur, and a residue containing between 50 and 65% of the initial aromatics, between 3 and 4.7% sulphur (corresponding to a concentration of sulphur compounds of approximately 12 to 21% by weight) some paraflins, naphthenes and aliphatic unsaturates, the concentration of the aliphatic unsaturates in the residue being low in comparison to their concentration in the initial oil. I have found that the concentration of aromatics in the distillate. can be controlled by introducing water into the olumn, the presence of larger amounts of water or steam in the distillation zone tending to increase the aromatics in the distillate, and decreasing the time necessary for distillation.

The distillate is condensed in a condenser 8 and a portion of the condensate may, if desired, be returned to the column through reflux lines 9 and ID and a valve II, which may be operated automatically to regulate the quantity of the reiiux, as by a thermostat or a pressure responsive device located in the column 5. The remaining portion of the condensate may be withdrawn at I2, together with the solvent, which may in certain cases be permitted to remain in the oil, as where it improves the characteristics of the product. In most cases the solvent will be removed from the oil, as by further distillation, in situations where azeotropes are not a hindrance, or, in any case, by flowing the condensate through a valve I3 and a chiller I4, and separating the resulting chilled condensate in a phase separator I5, which may, for example, be a centrifugal separator or a settling tank. The liquid phase which is rich in hydrocarbons is withdrawn from the separator at I6 and may be substantially free from solvent, in which case it is withdrawn at I'I, either as a separate product, or for blending with the product from the desulphurizing unit 36. 'I'he formation of a substantially solventfree hydrocarbon phase in the separator I5 may often be brought about by introducing water, or another highly polar substance into the condensate.

The further removal of solvent from the hydrocarbon phase withdrawn at I6 may be effected by extraction with a suitable solvent, like water, or by distilling this phase in a second distilling column I 8, which may be similar to the column 5, and provided with a reflux condenser I9 and reflux line 20 as shown, and operated to yield solvent-free hydrocarbons as a bottom product at 2| and the solvent, sometimes in ad'rnixture with hydrocarbons in form of a series of low boiling mixtures, as the top product at 22. This top product, if substantially free from hydrocarbons, may be fed into the column 5 or the tank 4; but since its composition will in most cases be similar to that of 'the distillate from the column 5, it will normally be passed through P. conduit 23 to the chiller I4 and the phase separator I5. It should be noted that the hydrocarbons contained in the top product from the column I8 are highly parainic, and it may in certain cases be desirable to treat this product in a separate chilling and phase separating unit.

The solvent phase from the separator I5 is withdrawn at 24 and contains 'a relatively low concentration of hydrocarbons. It may, therefore, be returned directly to the tank 4 through a conduit 25, as shown. If desired, it may be further concentrated in a distilling column, operated to produce substantially pure solvent as the bottom product which is returned to the tank 4, and a mixture of hydrocarbons and solvent as the top product. This top product may then be recycled to the chiller I4 and phase separator I5.

'Ihe sulphur compounds and those hydrocarbons which are preferentially soluble in the solvent employed, particularly aromatics, are concentrated in the bottom of the column 5 and are withdrawn at 26. 'I'he bottom product may be withdrawn continuously, or intermittently, and the column may be operated continuously or in a batch operation. The bottom product may or may not contain solvent, depending upon the boiling point of the solvent employed, and upon the compositions of the overall feed to the top product withdrawn from the column. The column may be operated so as to produce a solventfree bottom product while yet removing the greater portion or substantially all of the aliphatic unsaturates and substantially all of the parai'lins and naphthenes from the initial mixture by maintaining the ratio of the hydrocarbons which it is desired to remove in the distillate to the solvent the same in the feed through the conduit 6 as in the net top product withdrawn through the valves 21 and 28. This control may be effected in either of the following modes of operations:

a. 'I'he valves I and 3 are adjusted so that, in the resulting feed mixture the ratio of hydrocarbons to be removed in the distillate to the solvent is the same as the ratio of these components in the distillate withdrawn through the line 1.

b. The phase separator I 5 is provided with conduits 29, 30 and 3|, and valves 32 and 33, and a portion of one of the separated phases (or, contingently, a mixture of the two) is recycled to the column 5, either separately through an inlet 34, or together with the main reflux, through the plete disclosure of the step of vremoving unsatuproduct thus obtained.

rates from the voriginal oll, described 'several methods of operating the column)A and `o! fur,- ther treating the distillate to recoverl theA solvent, it should be noted that my process is in no way' limited to the specific types of `operations described, because my invention is primarily concerned with a process comprising the combination of steps of distilling in the presence'ofa solvent, and then desulphurizlng the bottom Inthe second stepof my process the lproduct withdrawn at vZlimay, if i desired, be further treated to remove the solvent, as by further distillation, or by the addition of a highly polar solubility reducing `agentllke water or aqueous methyl or ethyl alcohol, followed by chilling and stratiiication in a phase separator. The sulphur concentrate, with` or without solvent, however produced, substantially` free from aliphatlcimsaturateS, or with minor amounts of aliphatic..

unsaturates, is then treated in a desulphurizing unit 36. This treatment may, for example, com-c prise .the step `of contacting the` concentrate of sulphur compounds with sulphuric acid under conditions which favor-the solution of thiophenes but do not cause the sulphonatlon or dissolution of aromatics. For example, from 10 to 100% by volume of about from 70 to 95% su1phuric acid may be mixed with the sulphurconcentrateat a temperature from Aabout 20 C. to -i-40V C., after which acid is allowed to settle,` carrying the thiophenes and other sulphur compounds withA it in solution. I prefer, however, to employ from 25 to 75%,byvolume of sulphuric acid` ofbetween 87 and 92%' concentration `ata temperature between 0 Cfand +20 Except .at very low temperatures, Vconcentrations of acid above 95% `should not be used to avoid'the loss of aromatics. The acid treated distillation residue may, if desired, be washed with caustic and water, and/or, redistilled to eliminate sulphur compounds which havebeen chemically converted into oil-soluble `sulphur compounds boiling outside of the range ofthe untreated residue.

Some ofthe `aliphatic unsaturates` remaining in the distillation residue withdrawn atY 26 will be lost during this treatment, butthe loss is small, in view of the low concentration of these hydrocarbons in this residue. The loss of these` compounds, and,.to `a small extent, of aromatics,

can be further reduced, and the conditions of the. treatment rendered more nexible, ,by blending a highly parailinic distillate of the same o r a different boiling range with the residue, as tty` introducing at 31 a straight'run gasolineor paraflinic hydrocarbons-from the top product of the column I8. Such a distillate should contain less than about 5% and, preferably nounsaturated hydrocarbons. This expedient is particularly adunit 3l at 3l, and may, if desired, be blended with the distillate from the column 5 by opening the valve` 39, theA blended gasolinebeingwithdrawn at 40, although the distillate may be sepa-` rately withdrawn at 4| and 4utilized asa separate product. n

The treatment inthe unit 30 may, alternately, comprise a; selective hydrogenation treatment in the presenceo! a catalyst, like ,a mixture of Cos and. M003 or any other hydrogenation catalyst, under conditions causingl the removal oi thiophenes, but substantially `no-hydrogenation of aromatics.` Y

According to another method, ;-also involving reduction of sulphurcompounds, `the treatment in the unit v3|v may comprise the steps of conducting a vaporousmixture of carbon monoxide Iand the suiphur-containing distillation bottoms vaporize the sulphur compounds.

4I may also employ an oxidation treatment, the sulphur compounds being selectively oxidized into nonvolatile`l or water solubleI compoundswhich t can be` separated from the hydrocarbons by distillation ora water-wash. Any oxidizing agent may be employed for this purpose. For example, a 2% solution of `potassium permanganate can be contactedwith thedistillation bottoms. Dichromates,peracids or persalts may also be employed as oxidizingagents.` l According to still another alternative, the unit 36 may be operated to distill the sulphur -con centratein thepresence of ya metallic halide, such as aluminum chloride.,

Any other suitable desulphurizing treatment in the unit may, vhowevenbe substituted, these being well known inthe art,and it is ,understoodl thatmy process i's-not limited'to `any of the speciiicmodes of l operation i'orthe `unit 36 deseparately to my desulphurization treatment. By

a !oresighted selectiony of. boiling `ranges of the fractions it is often possible to isolate certain fractions requiring no treatment, and/orv which i may treated by ordinary methods,` such as washing with an alkalinegabsorbent for acidic gases, sodium plumbite treatment, or light sulphur" ric acid treatment, and to subject to my process only those 'fractions whicharediflicult to desulphurize by ordinary methods. For example, the fraction ofthe cracked distillate boiling between 50 and 80 C. may be given a wash' with caustic or potassiumcarbonate; the fraction boiling betweenY 80 and 150 C. maybe given alight sulphuric acid treatment; and thefraction boiling between 150 and 210 C. may be treated in the 'manner described above, eitheijdirectlyyor after a preliminary treatment to remove mercaptans and similar easily removable sulphur compounds. 1

Another purpose of this preliminary distillation is--tofeffect an increased emclency in the process of the invention. Thus, if the fractions to be treated have boiling ranges which are not substantially over 30 to 80 C., I am able to obtain a sharper separation between aliphatic unsaturates and sulphur compounds in the column 5, whereby both the quantity of sulphur compounds in the distillate and the quantity of aliphatic unsaturates in the residue are greatly reduced.

It is preferable to use a solvent having a boiling point which is sufilciently low to cause a mixture of said solvent and the highest boiling aliphatic unsaturate of the mixture introduced at I to boil at a temperature below the lowestl boiling sulphur compound to be removed in the residue. It is, therefore, possible to use solvents boiling at temperatures below the boiling range of the initial fraction, or at temperatures falling within or even above the said range, provided that at the operating pressure the solvent and the aliphatic unsaturates form low boiling mixtures which boil below the lowest boiling sulphur compound, or sufi'iciently low to produce a final top product of a desired sulphur content. Thus, while my process may be employed to cause the removal of substantially all of the sulphur from the distillate, it is also practiced when the sulphur content in the distillate is merely lowered.

I prefer to employ solvents boiling below the boiling range of the initial mixture and not more than C. below the initial boiling point thereof. Employing solvents with lower boiling points increases the ease of fractionation, but in most cases is accompanied with a low oil-solvent ratio in the distillate, the converse being true when higher boiling solvents are employed.

I may employ any solvent which has the above described boiling temperature characteristics, and which is a selective solvent for the sulphur compounds. I may use organic polar liquids of the type used in the liquid solvent extraction of hydrocarbon mixtures to eiect a separation between aromatic and parafiinic hydrocarbons, or between hydrocarbons of different degrees of saturation such as organic compounds, either aliphatic, carbocyclic, or heterocyclic, containing one or more oi' the following groups: NO2, -CN, -SCN, =CO, -CH2OH, =CHOH, ECOH, EC-CL NHL =O, ECCl, =CS, -COOH, -OR, -COOR, where R designates an alkyl radical, which may be either alphyl or aryl. Examples of these solvents are: nitrobenzene, benzonitrile, monochloraoetonitrile, phenyl thiocyanate, phenolthiocyanate, formate of pyridine, benzaldehyde, acetone, furfural, furfuryl alcohol, chlorisopropyl alcohol, diacetone alcohol, methyl alcohol, ethyl alcohol, aniline, methyl lactate, triacetin, diacetin, methyl nitrobenzoate, formic acid, acetic acid, ethylenchlorhydrin, glycol monoacetate, glycol diacetate, lactic acid nitrile, toluidine, chlor pyridine, pp' dichloroethyl ether, chlor aniline, sulphur dioxide, quinoline, isoquinoline, pyridine, chlorophenol, etc. My invention is not, however, limited to the specific solvents enumerated, nor to solvents containing the enumerated chemical groups, but may be used with-any selective solvent for sulphur compounds.

I have, however, found that when reilning sulphur-bearing hydrocarbon mixtures boiling within gasoline or kerosene ranges, superior results can be obtained by employing the following substances as solvents, and their use constitutes the preferred form of my invention: furfural, methyl cyanide, ethylene diamine, nitromethane and acetic anhydride.

I claim as my invention:

1. The process of rening an initial hydrocarbon fraction containing sulphur compounds and aliphatic unsaturated and aromatic hydrocarbons which comprises distilling the said initial fraction together with a preferential polar solvent for the said sulphur compounds in a distillation zone, maintaining the conditions of temperature and pressure in said zone to distill at least a substantial portion oi' the solvent together with unsaturated hydrocarbons, thereby forming a rectified top product derived from the combined mixture containing refined hydrocarbons and a. distillation bottom product in which the sulphur compounds are concentrated and containing a higher ratio of aromatics to aliphatic unsaturates than said initial fraction, and subjecting said bottom product to desulphurization by a chemical treatment.

2. The process according to claim 1 in which the solvent has a boiling point below the initial boiling point of the fraction to be treated, and not lower than 80 C. below said initial boiling point.

3. 'I'he process according to claim 1 in which the desulphurization treatment comprises the step of contacting the bottom product with sulphurlc acid under conditions causing the removal of thiophenes without substantially affecting aromatics.

4. The process according to claim l in which the desulphurization treatment comprises the step of subjecting the bottom product to selective hydrogenation to reduce thiophenes without substantially aiecting aromatics.

5. I'he process according to claim 1 in which the desulphurization treatment comprises the step of oxidizing the bottom product under conditions to selectively oxidize sulphur compounds without substantially affecting aromatics.

6. The process of refining an initial hydrocarbon fraction containing sulphur compounds and aliphatic unsaturated and aromatic hydrocarbons which comprises distilling the said initial fraction together with water and a preferential polar solvent for the said sulphur compounds in a distillation zone, maintaining the conditions of temperature and pressure in said zone to distill at least a substantial portion of the solvent together with aliphatic unsaturated hydrocarbons, thereby forming a rectiied top product derived from the combined mixture containing refined hydrocarbons and a distillation bottom product in which the sulphur compounds are concentrated and containing a higher ratio of aromatics to aliphatic unsaturates than said initial fraction, and subjecting said bottom product to desulphurization by a chemical treatment.

7. In a continuous process of refining an. initial hydrocarbon fraction containing sulphur compounds and aliphatic unsaturated and aromatic hydrocarbons, the steps of introducing said initial fraction and a preferential polar solvent for the said sulphur compounds into a distilling zone, maintaining within the zone counterflowing streams of vapors and reflux derived from the combined mixture, maintaining the conditions of temperature, pressure and reflux in said zone to distill at least a substantial portion of the solvent together with aliphatic unsaturated hydrocarbons, and forming a distillation bottom product in which the sulphur compounds are concentrated and containing a higher ratio of aromatics to aliphatic unsaturates than said initial fraction, removing the bottom product from the zone, blending the bottom product with a low boiling paraiiinic hydrocarbon distillate containing less than 5% unsaturates and subjecting the blended bottom product to desulphurization by a chemical treatment.

8. The process according to claim 'l in which the desulphurization treatment comprises the step of contacting the bottom product with sulphuric acid under conditions causing the removal of thiophenes without substantially affecting aromatics.

9. The process of reiining a hydrocarbon fraction containing sulphur compounds and aliphatic unsaturated and aromatic hydrocarbons which comprises distilling the said fraction together with a preferential polar solvent `for the said sulphur compounds in a distillation zone, maintaining the conditions of temperature and pressure in said zone to distill at least a substantial portion of the solvent together with aliphatic unsaturated hydrocarbons, thereby forming a rectied top product derived from the combined mixture containing refined hydrocarbons and a distillation bottom product in which the sulphur compounds are concentrated and containing a higher ratio of aromatics to aliphatic unsaturates than said initial fraction, subjecting the bottom product to desulphurization by a chemical treatment, separating the solvent from the hydrocarbons contained in the rectied top product, and blending the residual portion of the top product with the desulphurized bottom product.

10. The process for refining an initial cracked hydrocarbon fraction containing thiophenes,

which comprises distllling the said fraction together with a preferential polar solvent for thiophenes in a distillation zone, maintaining the conditions of temperature and pressure in said zone to distill at least a substantial portion of the solvent together with aliphatic unsaturated hydrocarbons, thereby forming a rectified top product derived from the combined mixture containing refined hydrocarbons and a distillation bottom product which contains a substantial portion' of the hydrocarbons together with a concentrate of the thiophenes and containing aliphatic unsaturates in a lower concentration than said initial fraction, and subjecting said bottom product to desulphurization by a chemical treatment.

11. The process according to claim 10 in which the distillation is carried out under conditions causing between and 90% of the initial fraction to be distilled.

12. 'I'he process according to claim 10 in which the initial cracked hydrocarbon fraction is a gasoline fraction boiling above about 140 C.

13. 'I'he process according to claim 10 in which the initial cracked hydrocarbon fraction is a gasoline fraction boiling above 140 C. and the solvent has a boiling point between and C.

14. The process according to claim 10 in which the solvent is iurfural.

15. The process according tov claim 10 in which the solvent is methyl cyanide.

- 16. 'I'he process according to claim 10 in which the solvent is ethylene diamine.

DONALD S. MCKITTRICK. 

